A thermoplastic polyurethane has been appreciated for its excellent properties, such as high toughness, and high resistance to oil and abrasion, etc. The marketability thereof is increasing. However, conventional thermoplastic polyurethanes have drawbacks in that they do not exhibit satisfactory resistance to light, so that they are likely to suffer from being yellowed when used for a prolonged period of time. For example, in the use of molded polyurethane products as articles having a nature of fashion, such as watchbands and the like, the discoloration to yellow is a serious problem. For alleviating this problem of the discoloration occurring with a lapse of time, many of such articles have generally been produced in black color. Further, it has recently been found that when a thermoplastic polyurethane is used as a material for producing articles which are used in direct contact with human skin, such as watchbands, the polyurethane is deteriorated due to the action of oleic acid which is a sebum component contained in sweat. Therefore, it has been recognized that the resistance to oleic acid is an important property for polyurethanes to be used as materials for such articles. Other examples of such articles include automobile parts, such as a steering wheel, gear shift lever and the like.
Furthermore, a polyurethane type coating composition has been appreciated in the field of leathers, such as a laminate synthetic leather and an impregnation composite artificial leather, with respect to the abrasion resistance, feeling to the skin, appearance and the like, so that it has been widely used as materials for a variety of articles, such as shoes, bags, belts, gloves, furniture such as sofas, and automobile seats. However, most of polyurethanes used for these articles are of polyester type or polyether type, thereby causing various problems. For example, in the case of furniture, such as sofas and office chairs, automobile seats and the like, which are used in direct contact with human bodies for a long period of time, the surfaces thereof are deteriorated due to the hydrolysis of the polyester type polyurethane resin, thereby causing the surfaces to be tacky. Furthermore, in extreme cases, a phenomenon is observed such that polyurethane resin layers are deteriorated, so that the resin layers are peeled off from substrates thereof, thereby rendering difficult the use of the articles for a long period of time. With respect to the polyether type polyurethanes, they have excellent resistance to hydrolysis. However, they are poor in resistance not only to oleic acid, but also to alcohol. The resistance to alcohol is an extremely important property in the above-mentioned uses, because, for example, alcohol is usually contained in hairdressing agents. That is, by the use for a long period of time, polyether type polyurethane resin layers formed On articles are deteriorated, thereby rendering the articles useless. For solving these problems, the use of polycarbonate type polyurethanes derived from 1,6-hexanediol has been proposed (see Unexamined Japanese Patent Application Laid-Open Specification Nos. 56-63079, 59-100778 and 59-100779).
With respect to these polycarbonate type polyurethanes derived from 1,6-hexanediol, the polyurethanes obtained by using aromatic diisocyanates as a diisocyanate have improved resistance to hydrolysis, alcohol, oleic acid, etc., as compared to polyester type polyurethanes and polyether type polyurethanes, however, they are poor in resistance to light, thereby causing a marked discoloration to yellow and a quality deterioration thereof when used for a long time. On the other hand, the polyurethanes obtained by using aliphatic diisocyanates as a diisocyanate are improved in resistance to light and hydrolysis, but are not satisfactory in resistance to alcohol and oleic acid, so that problems remain unsolved with respect to the practical use thereof.
In addition, with respect to polycarbonate type polyurethanes, polycarbonate diols are expensive, leading to a big problem. It has been desired to develop polycarbonate diols which have excellent properties and are less costly and which, therefore, can be advantageously used in substitution for conventional polycarbonate diols, obtained using 1,6-hexanediol, which have generally been commercially produced and put on the market.
In these situations, the present inventors have made extensive and intensive researches with a view toward obviating the above-mentioned drawbacks inevitably accompanying the conventional thermoplastic polyurethanes. As a result, it has unexpectedly been found that when a polytetramethylene carbonate diol having a number average molecular weight of from 500 to 10,000, which is prepared using 1,4-butanediol as a starting material, and an aliphatic diisocyanate are employed as a polycarbonate diol and an organic diisocyanate, respectively, a thermoplastic polyurethane which has excellent resistance to all of light, hydrolysis, alcohol and oleic acid can be obtained.
With respect to polyurethanes derived from polytetramethylene carbonate diols, there are only a few reports. For example, in "Polymer, Vol. 33, No. 7, pp. 1384-1390, 1992", a crosslinked type polyurethane derived from a polytetramethylene carbonate diol is reported. However, this type of polyurethane has poor moldability when molded by injection molding, extrusion molding, etc. When this type of polyurethane is used as a component for a coating composition, the resultant coating composition has high viscosity, so that it cannot be put into practical use. Furthermore, when the polyurethane is used in the form of an emulsion, the emulsion is disadvantageously unstable. On the other hand, with respect to thermoplastic polyurethanes derived from polytetramethylene carbonate diols, reports can be found in Italian Patent No. 710,940 and "Polymer (Korea), Vol. 14, No. 5, pp. 487-496, 1990", in both of which 4,4'-diphenylmethanediisocyanate is used as an organic diisocyanate. This type of thermoplastic polyurethane has a disadvantage in that it is poor in resistance to light.
With respect to a process for preparing a polytetramethylene carbonate diol, which is used as one of the raw materials for preparing the thermoplastic polyurethane of the present invention, from 1,4-butanediol, there are known methods in which as in the case of preparing a commercially available polyhexamethylene carbonate diol, a transesterification is effected between 1,4-butanediol and a carbonate, such as a dialkyl carbonate, a diaryl carbonate or an alkylene carbonate, as disclosed in Unexamined Japanese Patent Application Laid-Open Specification No. 55-56124, Examined Japanese Patent Application Publication No. 63-12896, etc. However, when the reaction is conducted in accordance with these methods, a carbonate reacts with 1,4-butanediol or with a terminal group of a produced polymer having a low molecular weight to form tetrahydrofuran (hereinafter referred to as "THF") as a by-product, leading to a problem such that it is difficult to increase the molecular weight of a polytetramethylene carbonate diol to be prepared and, hence, the productivity is lowered. To solve these problems, it has been attempted to lower the reaction temperature. However, this attempt is accompanied by disadvantages in that butylene carbonate is likely to be by-produced, and that due to the formation of this by-product, it is difficult to increase the molecular weight of a desired polytetramethylene carbonate diol, so that the problem on the productivity remains unsolved. Further, due to the formation of by-product THF, not only does a reaction under a reduced pressure becomes difficult to perform, but also it becomes necessary to employ an additional equipment, such as an absorption column, for lowering the concentration of THF which is released into the air through a vacuum pump. No commercially advantageous process has been available for preparing a polytetramethylene carbonate diol.
As a result of the studies by the present inventors toward developing a commercially advantageous process for preparing a polytetramethylene carbonate diol which is a starting material for the thermoplastic polyurethane of the present invention, it has unexpectedly been found that when the above-mentioned transesterification reaction is conducted in the presence of an acidic compound, occurrence of side reactions is markedly suppressed, so that it advantageously becomes possible to prepare a polytetramethylene carbonate diol having a relatively high molecular weight in high yield on a commercial scale.
The present invention has been completed, based on these findings described above.